IF the transaction never got confirmed, then Poloniex SHOULD morally refund your coins.

However, this might be against their terms of service. I'd suggest you contact them to see if they will recover the funds for you and put them back in (if not then you might want to keep emailing them until they get annoyed with you in the hope they will eventually solve the issue as it's an easy one to fix).

I have made a grave mistake. I have sent from a Bela poloniex account to an ETH coinbase account.

how can I access that transaction (which does not exist in the web) to recover the funds.

who can reverse or cancel said transaction

Did you accidentally withdraw BELA from your poloniex account to an ETH address? I'm first of all wondering that how this was even allowed to happen, as quite clearly, BELA addresses doesn't look compatible with ETH just from a simple glance.

Anyways, if the transaction is still at pending/processing, you should actually be able to cancel the transaction yourself from the UI on poloniex.

The transaction will most probably not go through, even if they do try to send funds to it. You will have to contact poloniex either way and see what they can do about the situation, since they are still in control of your coins if no transaction ID has been provided yet (meaning that coins still remain in their addresses). They may take some time to respond, though.